1. Field of the Invention
The present invention is situated in the field of proton exchange membrane fuel cells, well known under the acronym PEMFC.
It proposes a solution for limiting the phenomenon of corrosion of the carbon catalytic carrier that occurs at the cathode of such cells, and thus prolonging the service life thereof.
More precisely, the present invention is based on a reversal of the functioning of the cell.
2. Description of Related Art
PEMFCs are current generators the operating principle of which, illustrated in FIG. 1, is based on the conversion of chemical energy into electrical energy, by catalytic reaction of hydrogen and oxygen.
Membrane-electrode assemblies or MESs 1, commonly referred to as cell cores, constitute the basic elements of PEMFCs. They are composed of a polymer membrane 2 and catalytic layers 3, 4 present on either side of the membrane 2 and constituting respectively the anode and cathode.
The membrane 2 therefore separates the anodic 5 and cathodic 6 compartments. The catalytic layers 3, 4 generally consist of platinum nanoparticles supported by carbon aggregates. Gaseous diffusion layers 7, 8 (carbon fabric, felt, etc.) are arranged on either side of the MES 1 in order to provide the electrical conduction, the homogeneous distribution of the reactive gases and the discharge of the water produced by the reaction. A system of channels 9, 10 placed on either side of the MES brings in the reactive gases and discharges the water and excess gases to the outside.
At the anode 3, the decomposition of the hydrogen adsorbed on the catalyst produces protons H+ and electrons e31. The protons then pass through the polymer membrane 2 before reacting with the oxygen at the cathode 4. The reaction of the protons with the oxygen at the cathode leads to the formation of water and the production of heat (FIG. 2).
Improving the service life of PEMFCs constitutes a major factor for the use and development of cells for the mass market. This is why revealing and understanding phenomena of ageing of the core of such cells are at the present time essential.
It has been observed that the degradation of the materials of the electrodes concerns especially the cathodic active layer 4 (FIG. 3). Corrosion of the carbon catalytic carrier at the cathode, a well-known mechanism, is particularly detrimental for the cell.
The carbon carrier oxidises in accordance with the following reaction:C÷2H2O⇄CO2+4H++4e−  (1)
This degradation is accentuated when the cell is subjected to power cycles (J. P. Meyers and R. M. Darling J. Electrochem. Soc., 153 (8), A1432, 2006).
The potential of this reaction (1) is approximately 0.2 V/SHE. Given that the cathodic potential of a cell is generally greater than 0.2 V, this reaction always takes place.
Moreover, the oxygen present at the anode 3 is normally reduced by the hydrogen in the anodic compartment. However, during stop/start phases, power cycles, the formation of water plugs and the stoppage of the supply of hydrogen, the hydrogen is not sufficient to reach the oxygen. During these phases, the oxygen still present has recourse to other sources of protons and, in particular, to those produced by the oxidation of the cathodic carbon. The oxygen present at the anode 3 therefore acts as a proton (“proton pump effect”) that accentuates the corrosion of the carbon at the cathodic catalytic layer 4, and reaction (1) is then strongly moved to the right (FIG. 3):C÷2H2OCO2+4H÷+4e−  (2)
The degradation of the platinum at the cathode also participates in the reduction of the performance of the cell. One of the degradation mechanisms concerns the oxidation, dissolution and recrystallisation of the platinum.
Electrochemical maturation is another platinum degradation mechanism that leads to an increase in the size of the platinum particles.
In addition, degradation of the cathodic carbon carrier causes detachment of the platinum particles (A. A. Franco and M. Gérard J. Electrochem. Soc., 155 (4), B367, 2008) (Y. Shao, G. Yin and Y. Gao J. Power Sources, 171, 558, 2007).
Methods for increasing the service life of fuel cells have been proposed.
Thus one technical solution for limiting the corrosion of the carbon at the cathode, described in document JP 2006-278190, consists in introducing carbon dioxide (CO2) in air, at the cathode, and controlling the quantity thereof.
More recently, document FR 2 925 229 describes a solution based on a periodic reduction of the temperatures of the cell and humidifiers for a few hours, so as to maintain a stable relative humidity. This solution effectively significantly increases the service life of the cells but requires a temperature control device.
In addition, the introduction of a chemical compound in small quantities into the hydrogen (at the anode), such as CO, firstly limits the “proton pump” effect and therefore reduces the phenomenon of corrosion of the cathodic carbon, and secondly limits the degradation of the protonic conductive polymer. The service life of the cell is also significantly increased (A. A. Franco, M. Guinard, B. Barthe and O. Lemaire Electrochimica Acta., 54, 5267-5279, 2009).